Ive currently got this soundcard and kit for my computer... I have it connected using the SPDIF to my Sony 5.1 Amp.. However, I can configure it to output 5.1 stream during games, but it outputs 5.1 and DTS fine when playing a DVD movie through PowerDVD..

You don't specify but if you are after Dolby Digital 5.1 in your games then you need to get a soundcard that supports DD encoding as apposed to just decoding - as game sounds are positioned and mixed at run time.

It's not as simple as setting the speakers to 5.1 in the control panel and then selecting "Digital Output only" is it?

The 5.1 Live series had these options, so I assume that the Audigy will too

The official spdif format can only support 2 channels of PCM (PCM is uncompressed sound). Creative have a custom solution to support 6 channels of PCM, but of course that only works to their speakers.
Basically, for multichannel sound to work in PC Games, you either need a sound card which will compress on the fly to Dolby Digital, or a amp/speaker set which has a full set of analog inputs (commmon on speaker sets biased towards the PC market, and of course on higher end amps which like to have every conceivable connnection type).
To be fair, the analog approach is better quality. Compressing on the fly to DD to be decompressed a fraction of a second later is a bit silly quality wise.